1. Technical Field
The present invention relates to a crystal oscillator particularly suitable as a crystal oscillator having circuits other than a crystal resonator integrated on one chip.
2. Related Art
Conventionally, a temperature-compensated crystal oscillator (TCXO) is widely used in mobile communications apparatuses such as cellular phones.
FIG. 10 is a schematic diagram showing the composition of a conventional temperature-compensated crystal oscillator.
A TCXO 100 shown in FIG. 10 is composed of a crystal resonator 101 and a single-chip IC 102. A TCXO circuit section 103 excluding the crystal resonator 101 and an internal memory circuit 104 are integrated on the single-chip IC 102. The TCXO circuit section 103 is composed of a temperature-compensating voltage generator circuit 105 and a voltage control oscillator circuit 106. The temperature-compensating voltage generator circuit 105 generates a temperature-compensating voltage based on data from the internal memory circuit 104 and outputs the generated voltage to the voltage control oscillator circuit 106.
The voltage control oscillator circuit 106 is a voltage control type oscillator circuit equipped with a voltage variable capacitance element 107 and is considered capable of variable control of oscillation frequency by the temperature-compensating voltage coming from the temperature-compensating voltage generator circuit 105.
The internal memory circuit 104 stores, for example, data used to individually adjust parameters of a cubic coefficient and a primary coefficient of the voltage to the temperature, the voltage being generated at the temperature-compensating voltage generator circuit 105, in order to compensate the frequency temperature characteristics of the crystal resonator 101 expressed by a cubic curve.
JP-A-2004-356872 is an example literature on such a TCXO.
Recently, higher performance is demanded on the TCXO for the mobile phones and the like. Low phase noise characteristics and harmonic spectrum noise elimination are some of the strong demands being made in some cases. For example, it is requested that the phase nose be −135 dBc or less at an offset frequency of 1 kHz and that the harmonics be −15 dBc or less.
It is possible to improve the low phase noise characteristics and to eliminate the harmonic spectrum noise at the same time, if the output waveform of the crystal oscillator 100 is adjusted into a sine wave output. In this case, however, the crystal oscillator 100 becomes a unit only for a use that requires the sine wave output, and the usable range as a crystal oscillator becomes extremely limited.
In other words, with the conventional crystal oscillator, if the demand on its performance differs depending on the purpose of use, it is necessary to produce crystal oscillators equipped with waveform output circuits to satisfy different demands, and, thus, the conventional crystal oscillators stay costly and have not been put in general use.